1. Field of the Invention
This invention relates to a digital video display system and more particularly to such a display system having a video synthesizer providing a plurality of grayscale levels for a more natural display of characters.
2. Description of the Prior Art
There are commercially available today many examples of so called "video character displays" or "video terminals". These systems form representations of characters on the face of a cathode ray tube where each character is formed as a matrix of discrete points. Such a method of representation of characters predates electronics and may be found in embroidery, mosaics, and other disciplines employing a grid pattern. Such characters displayed on a cathode ray tube are of inferior readability unless the dot matrix allowed for each character is much larger than 5.times.7 or 7.times.9 which are the most commonly used matrices. Larger matrices require the electron beam to transverse many more points on each complete display fram, and thus require higher speed logic and cathode ray tube deflection circuits.
Although such systems may be called "video terminals", they are without exception incompatible with broadcast video standards and use a standard television set or monitor in such a crude fashion that half of its spatial and all of its gray-scale resolution is lost in forming an image.
The most important reason why designers have avoided the standard broadcast type of video raster is that it is interlaced. By producing the complete frame with two interlaced fields, broadcast video achieves twice the vertical resolution than can be achieved at that scan rate without interlace. However, dot-matrix characters appear to "flicker" when interlaced fields are used, particularly if the great majority of dots over any local region happen to fall within one or the other field. Flicker is a problem when interlace is employed not only for dot-matrix characters but also when there are very high contrast images. Thus, even when a standard video monitor is employed as the display device for a video terminal, it is used without interlace and is limited in its vertical resolution. Because this resolution is inadequate to display a large number of good quality dot-matrix characters, some systems must employ video rasters which are incompatible with standard video monitors. It is desirable to employ standard video monitors because of their economy due to mass production.
Such dot-matrix raster scan video displays are disclosed for example in the Cole U.S. Pat. No. 3,345,458. However, the concept of dot-matrix character generation for video display goes back to 1948 as evidence by the Dirks U.S. Pat. No. 2,972,016.
It is, then, an object of the present invention to provide a digital video display system which provides more natural display of characters and other information.
It is another object of the present invention to provide a digital video display system having interlaced scan for higher resolution but which display is "flicker" free.
It is still another object of the present invention to provide a video digital display system that can employ standard commercial video monitors.